Conventionally, various efforts have been made to increase the transmission capacity of a wireless communication system (hereinafter, may be referred to as a “system capacity”). For example, in the 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP LTE), discussion has been made on a technique for increasing the system capacity by utilizing a “small cell (SC)” in addition to a “macrocell”. A “cell” herein is a term representing a range covered by a wireless base station for transmission and reception of wireless signals by wireless terminals. The wireless base station and the cell indicate substantially corresponding concepts; therefore, in the explanation below, the “cell” may be interpreted as the “wireless base station” as appropriate. Further, the “macrocell” is a base station that can perform transmission with high transmission power, that is, a cell of a base station with a large coverage area. Furthermore, the “small cell” is a base station that performs transmission with low transmission power, that is, a cell of a base station with a small coverage area.
In the 3GPP LTE, as a configuration of a mobile communication system, a configuration including a plurality of small cells in a macrocell has been studied, for example. Further, a technique for simultaneously connecting a mobile station to a macrocell and a small cell has been studied. Furthermore, a technique for simultaneously connecting a mobile station to two different small cells has been studied. In this manner, communication performed by a mobile station simultaneously connecting to two different cells may be referred to as a dual connection (dual connectivity) in some cases. Alternatively, as the dual connectivity, there is a case where a base station directly connects to a higher-layer device. However, the configuration is not limited to the above, and, in general, the dual connectivity means that a terminal connects to and simultaneously communicates with a plurality of base stations to simultaneously transmit and receive different kinds of information to and from the base stations. In the explanation below, the dual connectivity is described; however, the same discussion can be applied to multiple connectivity such as triple or more connectivity. Therefore, the dual connectivity in the description below may be regarded as a concept including multiple connectivity, or the dual connectivity may be interpreted as multiple connectivity.
If a mobile station simultaneously connects to a macrocell and a small cell, for example, a control plane, which is a call to transmit L3 control information to set a transmission channel for transmitting data or to control a handover, is connected to the macrocell. Further, a data plane, which is a call to transmit and receive data, is connected to the small cell. Meanwhile, the control plane may be referred to as a C-plane or a signaling radio bearer (SRB). Further, the data plane may be referred to as a user plane, a U-plane, or a data radio bearer (DRB).
Furthermore, if a mobile station simultaneously connects to two different small cells, for example, it may be possible to employ a configuration in which the control plane is connected to one of the small cells and the data plane is connected to the other one of the small cells, or a configuration in which the data plane is connected to both of the base stations. As described above, in general, the dual connectivity means that a terminal connects to and simultaneously communicates with a plurality of base stations to simultaneously transmit and receive different kinds of information to and from the base stations.
In this manner, in the dual connectivity, a wireless base station to which a control plane is connected may be referred to as a primary wireless base station (cell). Further, a wireless base station (cell) to which a data plane for performing data communication in cooperation with the primary wireless base station (cell) may be referred to as a secondary wireless base station. Furthermore, these base stations may be respectively referred to as an anchor wireless base station (cell) and an assisting wireless base station (cell). Moreover, these base stations may be respectively referred to as a master wireless base station (cell) and a slave wireless base station (cell). Incidentally, in the case of the dual connectivity, to improve the communication characteristics, if a wireless communication terminal has a capability to receive pieces of data from two wireless base stations, it may be possible to employ a configuration in which each of the primary wireless base station and the secondary wireless base station outputs data to the wireless communication terminal. However, names of the wireless base stations are not limited to those described above. In general, as in a conventional LTE communication system, if a wireless base station that connects to and performs communication with both of a control plane and a data plane serves as a main base station, various names may be employed without departing from this intention. Hereinafter, for the sake of simplicity, terms “primary” and “secondary” are used.
As the configuration of the dual connectivity, various configurations have been proposed depending on at which of layers the data plane is separated.
For example, there is a configuration in which the data plane is separated in a stage before a packet data convergence protocol (PDCP) layer. Further, for example, there is a configuration in which the data plane is separated between the PDCP layer and a radio link control (RLC) layer. Furthermore, for example, there is a configuration in which the data plane is separated between the RLC layer and a medium access control (MAC) layer. The configurations are not limited to the above, and it is possible to employ a configuration in which separation is performed within a layer. For example, it may be possible to employ a configuration in which a part of the functions of the PDCP layer is implemented by a primary base station, and the rest of the functions of the PDCP layer is implemented by a secondary base station. The same applies to the RLC layer and the MAC layer.
If the configuration of the dual connectivity is employed, a primary wireless base station and a secondary wireless base station are connected by a wired link or a wireless link. Then, after the data plane is separated, data is sent to the secondary wireless base station via the link connecting the primary wireless base station and the secondary wireless base station.
Non Patent Document 1: 3GPP TR 36.932 V12.1.0 (2013-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Scenarios and requirements for small cell enhancements for E-UTRA and E-UTRAN (Release 12)
However, in the dual connectivity, in the secondary wireless base station, the MAC layer may acquire, from the RLC layer, pieces of data corresponding to a transmittable amount and transmit the pieces of data on the basis of an amount of transmitted data notified by a physical (PHY) layer. Alternatively, the MAC layer may acquire, from the RLC layer, pieces of data corresponding to a transmittable amount and transmit the pieces of data on the basis of transmittable data corresponding to a wireless quality notified by the PHY layer. In this case, the PDCP layer acquires a retention amount of data in the RLC layer, and thereafter sends pieces of data corresponding to the acquired retention amount to the RLC layer. In this configuration, the PDCP layer acquires the retention amount of data in the RLC layer via the link connecting the primary wireless base station and the secondary wireless base station. However, when the communication quality of the link connecting the primary wireless base station and the secondary wireless base station is not high, acquisition of the retention amount of data in the RLC layer by the PDCP layer may be delayed. In this case, the amount of data transmitted from the PDCP layer is not controlled appropriately, and data delivery from the PDCP layer may be delayed. Further, it is difficult to reflect the latest state of the retention amount of data in the RLC layer, so that a greater amount of data than the amount that can be stored in the RLC layer (buffering is possible) may be delivered from the PDCP layer to the RLC layer and the data may be discarded.